Basically, an electrochemical cell, or battery, is made up of two half-cells, each comprising an electronic conducting phase, or electrode, in contact with a second phase called an electrolyte, in which ionic conduction takes place. During discharge the electrolyte loses electrons to one of the electrodes thereby reducing that electrode. At the other electrode the electrolyte gains electrons, thereby oxidizing that electrode. The electrolyte associated with the positive electrode is referred to as the posilyte and the electrolyte associated with the negative electrode is referred to as the negalyte. In some cells the posilyte and negalyte are different solutions and therefore require a separator membrane to prevent the two solutions from physically mixing. In other types of cells the posilyte and the negalyte are the same, in which case the separator functions to physically separate the electrodes. These membranes should not, however, prevent ionic conduction between the posilyte and the negalyte.
Basic electrochemical cells may be classified as primary or secondary. Examples of primary battery systems are those having electrodes made of the active metal pairs mercury-zinc, silver-zinc, lead-zinc, copper-zinc, copper-magnesium, and silver-magnesium. Primary cells are generally not rechargeable. Examples of the most common secondary battery systems are those having electrodes made of the active metal pairs nickel-cadmium, silver-zinc and silver-cadmium. Secondary cells are rechargeable electrically by passing a current through the cell in a direction reversed from that of discharge. A common electrolyte used in both the primary and secondary cells is a 30 to 45% solution of potassium hydroxide.
In contrast to the above primary or secondary batteries, which employ pairs of active-metal electrodes held within the cell, the active material in the redox-battery is stored outside of the cell, in the form of pairs of solutions, or electrolytes. Storing the active material in this manner gives the battery a long shelf-life, and the capacity of the battery can be increased by increasing the amount of electrolyte. The heart of the system is the reactor cell where reduction and oxidation of the active species in the electrolyte occurs. The basic cell consists of two inert electrodes, which are commonly formed of carbon, separated by a separator membrane. On discharge, chemical energy is converted into electrical energy when the two reactive electrolytes are pumped through the cell compartments. During the charging stage, electrical energy is converted back to chemical energy and again, the two electrolytes are pumped through the cell compartments. Many such cells can be arranged into a stack to form a battery.
Redox batteries containing a variety of electrolyte pairs have been developed and are designated by the metal ions of the salts dissolved to form the posilyte and negalyte pairs. Some examples are iron (+3)/iron(+2) (U.S. Pat. No. 4,069,371; U.S. Pat. No. 4,053,684), zinc (+2)/iron(+2), chromium (+3)/iron(+3), titanium (+3)/iron(+2), titanium (+3)/bromine (-1), and zinc (+2)/bromine(-1).
Physically, the separator membranes prevent the reactive fluids from mixing and causing internal shorting-out of the battery. More specifically, the separator must allow the current-carrying ions to pass freely between compartments, while restricting other ions from passing which can cause self-discharge.
Micro-porous separators, which are characterized by relatively large-size pores in the membrane (0.01 to 0.1 microns), have been used in redox cells. The ion exchange separators disclosed herein exhibit a higher coulombic efficiency and an easier control of the hydraulic flow through the stacked cells (battery) in the redox system.
It is an object of this invention to provide an improved redox-type electrochemical cell incorporating separator membranes which have low electrolytic resistance.
It is a further object to provide an improved redox-type electrochemical cell incorporating a membrane which exhibits a high selectivity against positive ion migration during operation of a cell and particularly which exhibits selectivity against ferric ion migration during operation of an iron (+3)/iron (+2) redox cell.
It is a further object of this invention to provide an improved redox-type electrochemical cell incorporating a membrane which exhibits long-term stability in acidic electrolytes.
It is a further object of this invention to provide an improved redox-type electrochemical cell incorporating a membrane which resists fouling during operation.
It is a further object of this invention to provide an improved redox-type electrochemical cell incorporating a membrane which exhibits minimal transport of bulk fluids.
Still other objects and advantages of the present invention will be obvious and apparent to those skilled in the art from the specification and the appended claims.